Since the 386SL processor was introduced by the Intel Corporation, System Management Mode (SMM) has been available on 32-bit Intel Architecture (IA32) processors as an operation mode hidden to operating systems that executes code loaded by the basic input/output system (BIOS) or firmware. The mode is deemed “hidden” because SMM operations occur independently of the operating system (OS) and software applications.
IA32 processors are enabled to enter SMM via activation of a System Management Interrupt (SMI) signal. A similar signal called the Processor Management Interrupt (PMI) signal that is roughly analogous to the SMI signal is used for Itanium™-class processors, also available from Intel Corporation. For simplicity, both SMI and PMI signals can be referred to as xMI.
To date, most BIOS implementations that leverage the SMM capability simply register a monolithic section of code that is created during the build of the BIOS to support a specific function or set of functions particular to systems that use the BIOS. There is no provision in today's systems for the registration or execution of third-party SMM code, thus allowing no extensibility to the SMM framework. Such extensibility is often desired. For example, if the functions provided by the SMM code provided by the original equipment manufacturer (OEM) or the BIOS vendor for a given platform is insufficient, a developer or value-added reseller (VAR) has to either license the existing code from the BIOS vendor or OEM and attempt to graft their own logic into their implementation of SMM code.
In addition, today's implementations on IA32 processors are restricted to the 16-bit mode of the processor, thus limiting the size of the code and the possible leveraging of 32-bit or 64-bit software engineering techniques. In general, BIOS updates for SMM functions are problematic to effect and because the OS already has a hardware extensibility mechanism via its own driver model, BIOS vendors and OEMs are less motivated to provide these types of BIOS updates.